1. Field of the Invention
The present invention relates to a method of detecting a pneumatic pressure drop of the respective tires provided for a four-wheel vehicle, and a tire pressure drop detecting device for carrying out this method.
2. Description of the Related Art
As one of safety devices for a four-wheel vehicle such as automobile, truck, etc., tire pressure drop detecting devices have recently been proposed, and some of them have been put to practical use.
The tire pressure drop detecting device has been developed because it's importance is recognized due to the following reason. That is, when the pneumatic pressure drops, the temperature of the tire increases due to an increase of deflection. When the temperature becomes high, the strength of a polymer material used for the tire is lowered and the tire is likely to burst. Normally, even if the tire is deflated by a pressure of approximately 0.5 atm., a driver is often unaware of the deflation so that a device capable of detecting the deflation has been desired.
As one of methods of detecting a tire pressure drop, for example, there is a method of utilizing a difference in the respective rotational angular velocities F.sub.1, F.sub.2, F.sub.3 and F.sub.4 (referred to as a "rotational angular velocity F.sub.i " hereinafter) of four tires W.sub.1, W.sub.2, W.sub.3 and W.sub.4 (referred to as a "tire W.sub.i hereinafter) provided for the vehicle. The tires W.sub.1 and W.sub.2 correspond to right front and left front tires, and the tires W.sub.3 and W.sub.4 correspond to right rear and left rear tires, respectively.
According to this method, the rotational angular velocities F.sub.i of the tires W.sub.i are detected every predetermined sampling period, based on a signal to be outputted from a wheel speed sensors mounted on respective tires. When effective rolling radiuses of four tires W.sub.i are the same and, at the same time, the vehicle is traveling linearly, four detected rotational angular velocities F.sub.i are the same. The effective rolling radius is a value obtained by dividing by 2.pi. a travel distance of the vehicle while the tire makes one free revolution.
The effective rolling radius of the tire W.sub.i varies depending on the change in pneumatic pressure of the tire W.sub.i, That is, when the pneumatic pressure of the tire W.sub.i drops, the effective rolling radius becomes smaller than that at a normal internal pressure. Accordingly, the rotational angular velocity F.sub.i of the tire W.sub.i whose pneumatic pressure drops becomes larger than that at a normal internal pressure. Therefore, the pressure drop of the tire W.sub.i can be detected based on the difference in rotational angular velocities F.sub.i of four tires.
One embodiment of a method of determining the pressure drop of the tire W.sub.i based on the difference in rotational angular velocities F.sub.i is as shown in the following equation (1) (e.g. see Japanese Laid-Open Patent Publication Nos. 63-305011 or 4-212609). ##EQU1##
For example, when it is assumed that the effective rolling radiuses of four tires W.sub.i are the same, the rotational angular velocities F.sub.i are the same (F.sub.1 =F.sub.2 =F.sub.3 =F.sub.4). Accordingly, the judged value D becomes 0. Then, threshold values D.sub.TH1 and D.sub.TH2 are set (D.sub.TH1, D.sub.TH2 &gt;0). If the condition shown in the following expression (2) is satisfied, it is judged that one of the tires W.sub.i has a pneumatic pressure drop. If this condition is not satisfied, it is judged that none of the tires W.sub.i has a pneumatic pressure drop. EQU D&lt;-D.sub.TH1 or D&gt;D.sub.TH2 ( 2)
By the way, the effective rolling radius of the tire W.sub.i varies depending on factors such as variation in tire radius at the time of producing the tires W.sub.i, from that indicated by the design specification (referred to as an "initial difference" hereinafter), speed of the vehicle, turning radius of the vehicle, front/rear acceleration exerted on the vehicle, lateral acceleration exerted on the vehicle, etc.
Accordingly, even if the pneumatic pressure of the tire W.sub.i is normal, the effective rolling radius of the tire W.sub.i varies depending on the above variable factors. Thereby, the rotational angular velocity F.sub.i of the tire W.sub.i varies. As a result, the above judged value D is likely to become a value other than 0. Therefore, there is a fear of causing a wrong detection, that is, it is judged that the pneumatic pressure drops in spite of no pressure drop. Therefore, it is necessary to exclude the influence of the above factors other than pressure drop which vary the effective rolling radius of the tire, so as to detect the pressure drop with high accuracy.
The technique for excluding the influence of factors including the speed, turning radius, front/rear acceleration and lateral acceleration of the vehicle from the tire pressure drop detection is, for example, disclosed in Japanese Laid-Open Patent Publication No. 7-164842 (laid-open on Jun. 27, 1995) by one of the inventors of the present application. In the technique disclosed in this document, the corrected value C can be determined according to the following equation (3) using the speed V, turning radius R, front/rear acceleration FRA and lateral acceleration LA of the vehicle determined based on the output of the wheel speed sensor provided for each tire W.sub.i of the four-wheel vehicle. The judged value D which was previously calculated is corrected as shown in the following equation (4), using this correction value C, thereby determining the judged value D' after correction. EQU C=(K1.times.V.times.FRA.times.LA)+(K2.times.V.times.LA)+(K3.times.1/R)+K4(3 ) EQU D'=D-C (4)
Factors K1 to K4 of the corrected value C respectively indicate constants which are obtained empirically under the condition where it has already been known that all tires W.sub.i have a normal internal pressure and a certain range of the front/rear acceleration FRA and lateral acceleration LA act on the vehicle and, at the same time, the vehicle is traveling along a path having a turning radius R.
In such way, the previously calculated judged value D is subjected to the correction suited to variable factors including the speed V, turning radius R, front/rear acceleration FRA and lateral acceleration LA of the vehicle. Thereby, the judged D' in which the influence of the above variable factors have been excluded should be obtained, as a matter of course.
By the way, FIG. 7, FIG. 8 and FIG. 9 are graphs illustrating a relationship between the judged value D which was measured, when the vehicle was traveling at constant speed (various speeds V) where all tires W.sub.i had a normal internal pressure, and a product V.sub.G of the speed V and lateral acceleration LA of the vehicle at that time, respectively. FIG. 7, FIG. 8 and FIG. 9 correspond to the cases where the vehicle traveled along pathes in which the turning radiuses R of the vehicle become R.sub.1, R.sub.2 and R.sub.3 (e.g. R.sub.1 =40 m, R.sub.2 =57 m, R.sub.3 =150 m), respectively.
In the respective figures, when the method of least squares is applied using a primary equation: D=.alpha.V.sub.G +.beta., the following equations (5), (6) and (7) can be obtained. EQU D=.alpha..sub.1 V.sub.G +.beta..sub.1 ( 5) EQU D=.alpha..sub.2 V.sub.G +.beta..sub.2 ( 6) EQU D=.alpha..sub.3 V.sub.G +.beta..sub.3 ( 7)
where .alpha..sub.1, .alpha..sub.2 and .alpha..sub.3 correspond to the factor K2 in the equation (3), respectively. When R.sub.1 is 40 m, R.sub.2 is 57 m, and R.sub.3 is 150 m, .alpha..sub.1, .alpha..sub.2 and .alpha..sub.3 become 0.029, 0.025 and 0.008, respectively. As shown in FIG. 10, this factor K2 varies depending on the change in turning radius R of the vehicle.
However, in the above equation (3), a trial of excluding the influence of the turning radius R of the vehicle from the tire pressure drop detection is made exclusively by the term of (K3.times.1/R) as the third term of the right side. That is, regarding the second term including the factor K2, the influence of the turning radius R of the vehicle is not taken into consideration.
In addition, it has also been understood that the first term of the right side of the above equation (3) is influenced by the above variable factors, particularly turning radius R of the vehicle.
Therefore, even if the judged value D is corrected by the correction value C determined by the above equation (3), the influence of the above variable factors can not be excluded, sufficiently, and hence the tire pressure drop is not detected accurately.